The present invention relates to diene rubber compositions reinforced with a white filler, which are intended, in particular, for the manufacture of tires or semi-finished products for tires, in particular, treads for these tires.
Since fuel economies and the need to protect the environment have become priorities, it has become desirable to produce elastomers with good mechanical properties and as low a hysteresis as possible so that they can be used in the form of rubber compositions for the manufacture of various semi-finished products involved in the constitution of tires, including inter alia, underlayers, calendering or sidewall rubbers, or treads, and to obtain tires with improved properties, in particular, reduced rolling resistance.
To achieve such an objective, numerous solutions have been proposed, such as those essentially concentrating on the use of elastomers modified by means of agents, such as coupling, starring or functionalizing agents, with carbon black as the reinforcing filler, with the aim of obtaining a good interaction between the modified elastomer and the carbon black. It is generally known that, in order to obtain the optimum reinforcement properties imparted by a filler, the filler should be present in the elastomeric matrix in a final form which is both as finely divided as possible and distributed as homogenously as possible. Currently, such conditions may only be obtained insofar as the filler has a very good ability, first, to be incorporated into the matrix during mixing with the elastomer and to disagglomerate and, second, to be dispersed homogenously in this matrix.
It is well known that while carbon black has such abilities, it is generally not true of white fillers. For reasons of mutual attraction, white filler particles have an irritating tendency to agglomerate together within the elastomeric matrix. These interactions have the harmful consequence of limiting the dispersion of the filler and hence its reinforcing properties to a level substantially lower than which it would be theoretically possible to achieve if all the (white filler/elastomer) bonds that could be created during the mixing operation were in fact obtained. These interactions further tend to increase the consistency of the rubber compositions and, therefore, to make them more difficult to work (xe2x80x9cprocessabilityxe2x80x9d) in the uncured state than if carbon black were used as filler.
However, interest in rubber compositions reinforced with white filler was greatly revived with the publication of European Patent Application EP-A-0 501 227, which discloses a sulphur-vulcanizable diene rubber composition, reinforced with a special precipitated silica of the highly dispersible type, which makes it possible to manufacture a tire or tread with substantially improved rolling resistance, without adversely affecting the other properties, in particular those of grip, endurance and wear resistance. Application EP-A-0 810 258 discloses a novel diene rubber composition reinforced with another special white filler, i.e., a specific alumina (Al2O3) of high dispersibility, which also makes it possible to obtain tires or treads having an excellent compromise of contradictory properties.
Although the use of these specific, highly dispersible silicas or aluminas as reinforcing fillers, whether or not as the major component, has reduced the difficulty of processing the rubber compositions that contain them, such compositions are still more difficult to process than rubber compositions filled conventionally with carbon black.
In particular, it is necessary to use a coupling agent, also known as a bonding agent, the function of which is to provide the connection between the surface of the white filler particles and the elastomer, while also facilitating the dispersion of this white filler within the elastomeric matrix.
The term xe2x80x9ccoupling agentxe2x80x9d (white filler/elastomer) is understood by those skilled in the art to mean an agent capable of establishing a sufficient chemical and/or physical connection between the white filler and the elastomer. Such a coupling agent, which is at least bifunctional, has, for example, the simplified general formula xe2x80x9cYxe2x80x94Txe2x80x94Xxe2x80x9d, in which:
Y represents a functional group (xe2x80x9cYxe2x80x9d function) which is capable of bonding physically and/or chemically with the white filler, such a bond being able to be established, for example, between a silicon atom of the coupling agent and the hydroxyl (OH) surface groups of the white filler (for example, surface silanols in the case of silica);
X represents a functional group (xe2x80x9cXxe2x80x9d function) which is capable of bonding physically and/or chemically with the elastomer, for example by means of a sulphur atom;
T represents a hydrocarbon group that links Y and X.
Such coupling agents are not to be confused with simple agents for covering the white filler which, in known manner, may comprise the above-cited Y function which is active with respect to the white filler but are devoid of the X function which is active with respect to the elastomer.
Silica/elastomer coupling agents, in particular, have been described in a large number of documents, the best known being bifunctional alkoxysilanes.
In patent application FR-A-2 094 859 it was proposed to use a mercaptosilane for the manufacture of tire treads. It was quickly shown, and is today well known, that the mercaptosilanes, particularly xcex3-mercaptopropyltrimethoxysilane or xcex3-mercaptopropyltriethoxysilane, are capable of giving excellent silica/elastomer coupling properties, but that these coupling agents cannot be used industrially because of the high reactivity of their -SH functions. Such high reactivity very rapidly results in premature vulcanization during the preparation of the rubber composition in an internal mixer, known as xe2x80x9cscorchingxe2x80x9d, which results in very high Mooney plasticity values and, ultimately, results in rubber compositions which are virtually impossible to work and process industrially. FR-A-2 206 330 and U.S. Pat. No. 4,002,594 illustrate the impossibility of industrially using such coupling agents bearing -SH functions and the rubber compositions that contain them.
To overcome this drawback, it has been proposed to replace these mercaptosilanes by polysulphurized alkoxysilanes, in particular bis-trialkoxy(C1-C4)silylpropyl polysulphides, such as described in numerous patents or patent applications (see for example FR-A-2 206 330, U.S. Pat. Nos. 3,842,111, 3,873,489, 3,978,103 or 3,997,581). Preferred polysulphides, include bis-3-triethoxysilylpropyl disulphide (xe2x80x9cTESPDxe2x80x9d), more particularly, bis-3-triethoxysilylpropyl tetrasulphide (xe2x80x9cTESPTxe2x80x9d), the latter being currently regarded as the product which, for vulcanized rubber compositions filled with silica, gives the best compromise in terms of resistance to scorching, processability and reinforcing power.
During the course of research, the Inventor discovered that the use of a zinc dithiophosphate in a small quantity, in combination with a guanidine derivative, had the unexpected effect of activating the coupling function of the polysulphurized alkoxysilanes, resulting in increased effectiveness of the latter.
Due to this activation, it is possible in to substantially reduce the quantity of polysulphurized alkoxysilanes which are usually used in rubber compositions. This is particularly advantageous because these alkoxysilanes are very expensive and also have to be used in a large quantity, generally in the order of two to three times more than the quantity of xcex3-mercaptopropyltrialkoxysilanes necessary to obtain levels of equivalent coupling properties. See, e.g., U.S. Pat. Nos. 5,652,310, 5,684,171 and 5,684,172. Thus, the present invention can significantly reduce the overall cost of the rubber compositions, as well as that of the tires and/or tire components containing them.
The present invention relates to a rubber composition usable for the manufacture of tires, comprising as base constituents one or more diene elastomer(s), a reinforcing white filler, a polysulphurized alkoxysilane as coupling agent (white filler/diene elastomer), with which are associated a zinc dithiophosphate and a guanidine derivative.
Another subject of the invention is the use of a rubber composition according to the invention for the manufacture of rubber articles, in particular tires or semi-finished products intended for such tires, these semi-finished products being selected from the group consisting of treads, underlayers intended, for example, to be positioned beneath these treads, crown plies, sidewalls, carcass plies, beads, protectors, inner tubes and airtight internal rubbers for tubeless tires. The invention relates more particularly to the use of such a rubber composition for the manufacture of the sidewalls or treads, owing to its good hysteretic properties.
The invention also relates to a process for the preparation of a rubber composition according to the invention, this process being characterized in that there are incorporated by kneading into one or more diene elastomer(s), a white filler as reinforcing filler, a polysulphurized alkoxysilane coupling agent (white filler/diene elastomer), a zinc dithiophosphate and a guanidine derivative. The invention also encompasses the rubber composition prepared by this process.
The composition according to the invention is particularly suitable for the manufacture of treads for tires intended to be fitted on passenger vehicles, 4xc3x974 vehicles, vans, two-wheelers and heavy vehicles, aircraft, or construction, agricultural or handling machinery, these treads being able to be used in the manufacture of new tires or for recapping worn tires.
The subject of the invention is also these tires and these semi-finished rubber products themselves, in particular the treads, comprising a rubber composition according to the invention.
The invention also relates, per se, to a coupling system (white filler/diene elastomer) for a diene rubber composition reinforced with a white filler, usable for the manufacture of tires, said system being formed by the association of a polysulphurized alkoxysilane, a zinc dithiophosphate and a guanidine derivative.
The subject of the invention is also the use of such a coupling system (white filler/diene elastomer) in a rubber composition for a tire.
The invention is further directed to of a zinc dithiophosphate and a guanidine derivative in combination, in rubber compositions reinforced by a white filler usable for manufacturing tires, for activating the coupling function (white filler/diene elastomer) of the polysulphurized alkoxysilane.
The invention and its advantages will be readily understood in the light of the description and examples of embodiment which follow, and of the figures relating to these examples, which show curves of the variation of modulus as a function of elongation for different diene rubber compositions, whether or not in accordance with the invention.